1. Field of the Invention
The present invention is directed to a method for magnetic resonance (MR) imaging that can be particularly utilized for the implementation of sensitive magnetic resonance measurements wherein a fluctuating transmission field strength has a negative influence.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for acquiring images of the inside of the body of a living examination subject. For the implementation of magnetic resonance tomography, a basic field magnet generates a static, relatively homogeneous basic magnetic field. During the registration of magnetic resonance images, rapidly switched gradient fields for location coding are superimposed on this basic magnetic field, these gradient fields being generated by gradient coils. Radio-frequency pulses having a defined transmission field strength B1 for triggering magnetic resonance signals S are emitted into the examination subject with radio-frequency transmission antennas. The magnetic resonance signals produced with these radio-frequency pulses are picked up by radio-frequency reception antennas. The magnetic resonance images of the examined subject region of the examination subject are produced on the basis of these magnetic resonance signals received with the reception antennas. Each picture element in the magnetic resonance image is allocated to a small body volume, referred to as the voxel. The brightness or intensity value of the picture element is dependent on the signal amplitude of the magnetic resonance signal received from this voxel. The intensity of the magnetic resonance signal S is in turn dependent on the intensity of the emitted field B1 of the radio-frequency transmission antennas, among other things.
The exciting transmission field strength B1 cannot be kept precisely constant due to unavoidable fluctuations in the transmission power, for example due to temperature fluctuations or disturbances in the amplifier as well as the properties of the transmission antenna, for example due to heating or a change in capacitance given movements on the part of the patient. The effects of a fluctuating transmission field strength are disturbing particularly when sensitive measurements wherein extremely minute signal differences in the magnetic resonance signals are of significance. An example of such a measurement is functional magnetic resonance imaging (fMRI) with which information about the brain activity in humans and animals can be obtained. In functional magnetic resonance tomography, magnetic resonance exposures of the subject volume to be examined, the brain of a patient, are made at short time intervals. A stimulus-specific neural activation can be detected and spatially localized by comparing the signal curve measured with the means of functional imaging for each volume element of the subject volume to the time curve of a model function. Since the minute changes of the received magnetic resonance signal triggered by physiological events must thereby be detected, functional magnetic resonance imaging requires an extremely high stability of the magnetic resonance system. Fluctuations in the transmission field strength can greatly degrade the analytical validity of the measurements.
Two different concepts are currently known for countering this known problem of the fluctuating transmission field strength in measurements of functional magnetic resonance tomography. One approach employs class-A transmission amplifiers as the antenna transmission amplifiers. These offer a high stability of the amplification in order to achieve an optimally constant transmission field strength during the measurement. These transmission amplifiers, however, are extremely complicated technically and also exhibit a poor efficiency.
It is also known to place a reference object in the form of a phantom object next to the examination subject in the measurement field or measurement volume (FOV=field of view) during the measurement, a reference signal being obtained from this reference object. The magnetic resonance signals received from the volume elements (voxels) of the examination subject are then referenced to this reference signal in order to correct fluctuations of the transmission field strength in the magnetic resonance signals. This technique, however, is only meaningful given a repetition time TR of the excitation that is far longer than the longest longitudinal relaxation time T1 (usually about 1 second) occurring in the tissue of the examination subject, since the signal S of a volume element is then proportional to sin(α), whereby α˜B1, so that S correlates directly with the transmission field strength B1. Due to the saturation of the spin, the maximum of the signal then appears at α<90°, and the relationship between the signal S and the transmission field strength B1 becomes dependent on the type of tissue of the respective volume element. In this case, the longitudinal relaxation time T1 of the phantom cannot be representative for the overall measurement volume. Moreover, a different mix of a number of signal-generating materials is present within each and every volume element, so that a simple allocation of the macroscopic tissue type to an effective T1 generally is not possible.